1. Field of the Invention
The present invention relates to a flame-retardant polypropylene resin composition. More specifically, it relates to a flame-retardant polypropylene resin composition mainly comprising a polypropylene resin, giving off neither a corrosive gas nor a poisonous gas, keeping up good processability, and providing molded articles in which mechanical strength is stable and flame retardance is excellent.
2. Description of the Prior Art
Heretofore, polypropylene resins were often used in the field of household electric appliances and in other fields of buildings, interior decorations, automobile parts and the like, because of excellent processability, chemical resistance, weathering resistance, electrical properties and mechanical strength.
Originally, the polypropylene resins had the potential to burn, but with the expansion of their use applications, performance as flame-retardant materials was required for the polypropylene resins, and the performance level under demand has heightened year by year. In order to meet this demand, a variety of flame-retardant polypropylene resin compositions have been suggested.
For example, Japanese Patent Laid-open Publication Nos. 92855/1978, 29350/1979, 77658/1979, 26954/1981, 87462/1982 and 110738/1985 disclose compositions prepared by adding, to polypropylene resins, hydrous inorganic compounds (e.g., magnesium hydroxide, aluminum hydroxide, barium sulfate, magnesium oxide, oxides of antimony, hydrotalcite and the like); Japanese Patent Publication No. 30739/1980 discloses a composition prepared by adding, to a polypropylene resin, a polyethylene having a melt index of 0.01 to 2.0, decabromo diphenyl ether (or dodecachlorododecahydromethanodibenzocyclooctene) and at least one inorganic filler selected from the group consisting of powdery talc, kaolinite, sericite, silica and diatomaceous earth; and Japanese Patent Laid-open Publication No. 147050/1984 discloses a composition prepared by adding ammonium phosphate (or phosphoric acid amine) and the oligomer (or polymer) of a 1,3,5-triazine derivative to a polypropylene resin.
However, in the composition prepared by adding a hydrous inorganic compound, for example, magnesium hydroxide to a polypropylene resin, a great deal of the hydrous inorganic compound must be added thereto in order to obtain high flame retardance. As a result, the moldability of the composition deteriorates, and products manufactured by molding the composition are poor in mechanical strength.
Furthermore, the composition containing a halogen compound inconveniently gives off a corrosive gas and a poisonous gas, when processed and burnt, though moldability is not so bad and molded articles of this composition have suitable mechanical strength and high-level flame retardance.
The above-mentioned flame-retardant polypropylene resin composition disclosed in Japanese Patent Laid-open Publication No. 147050/1984 has good moldability and gives off a lower volume of corrosive and poisonous gases when processed and burnt, and molded articles of this composition have suitable mechanical strength.
However, according to a test corresponding to a vertical burning test in "Flammability Test of Plastic Materials for Instrument Parts" of UL Subject 94 (Underwriters Laboratories Incorporation) (hereinafter referred to as UL 94 vertical burning test), this composition takes a flame-retardant rank of V-0 in a wall thickness of 1/8 inch and V-2 in a wall thickness of 1/32 inch which is the requirement of high flame retardant. In a 5V test of the UL 94 vertical burning test in which higher flame retardance is required, droplets of the above-mentioned composition drop, and melted, cut and burnt pieces of this composition fall (hereinafter these droplets and pieces are referred to as drips, and charcteristics by which the drips are formed are referred to as drip properties). Therefore, the suggested flame-retardant composition can hardly achieve the flame-retardent level of 5V in a wall thickness of 1/8 inch in the UL 94 vertical burning test which is the requirement of high flame retardance. In addition, even if an olefinic synthetic rubber or elastomer which can usually be used for the improvement in impact strength of a polypropylene is added to this kind of composition, impact properties such as Izod impact strength and the like are scarcely improved.